Printed circuit board interconnecting structure with compliant cantilever interposers

ABSTRACT

An interconnecting structure for interconnecting two electronic modules. The structure includes a dielectric substrate having a copper trace deposited on the lower surface thereof, and a copper pad disposed on the upper surface of the substrate directly above one end of the trace. A first copper plate-up area deposited on the pad, and a second copper plate-up area is deposited on the distal end of the trace. A slot, semi-circumscribing the pad and extending on both sides of the trace toward the distal end of the trace, is cut through the substrate to allow the proximal end of the trace to be displaced in a cantilevered manner below the lower side of the substrate when a force is applied to the pad.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 60/986,947 filed Nov. 9, 2007, the disclosure of which isincorporated herein by reference.

SUMMARY

An interconnecting structure for interconnecting two electronic modulesis disclosed. The structure includes a dielectric substrate having acopper trace deposited on the lower surface thereof, and a copper paddisposed on the upper surface of the substrate directly above one end ofthe trace. A first copper plate-up area deposited on the pad, and asecond copper plate-up area is deposited on the distal end of the trace.A slot, semi-circumscribing the pad and extending on both sides of thetrace toward the distal end of the trace, is cut through the substrateto allow the proximal end of the trace to be displaced in a cantileveredmanner below the lower side of the substrate when a force is applied tothe pad.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an exemplary diagram showing a partial top view of a printedcircuit board including a plurality of interposers;

FIG. 1B is an exemplary diagram showing a top view of one of theinterposers of FIG. 1A;

FIG. 2 is a flowchart showing an exemplary process for manufacturing aprinted circuit board in accordance with the present method;

FIG. 3A is an exemplary diagram showing a side view of two printedcircuit boards, each having an offset cantilever in a non-compressedcondition;

FIG. 3B is an exemplary diagram showing the circuit boards of FIG. 1Awith the cantilevers in a compressed condition;

FIG. 4 is an exemplary diagram showing current flow through aninterposer;

FIG. 5 is an exemplary perspective view of the interposer of FIGS. 1A,1B, 3A and 3B, showing the U-cut;

FIG. 6 is an exemplary diagram showing a cutaway view of the interposerof FIG. 5; and

FIG. 7 is an exemplary diagram showing a configuration in which aninterposer includes two U-cuts.

DETAILED DESCRIPTION

A dual-sided printed circuit (PC) board including an interconnectingstructure for electrically coupling two electronic modules and acorresponding method for manufacturing the structure is disclosedherein. This interconnecting structure (hereinafter ‘interposer’)includes a conductive trace, on the PC board substrate, around one endof which a ‘U’-shaped slot is cut through the substrate. Undercompression, such as when compressed by an above device under test and atest board below, or during electrical mating of two circuit boards,this U-shaped slot allows the interposer to be displaced to form acantilevered structure.

This cantilevered structure provides for mating compliance such that theinterposers can accommodate slight pad or board height variations(2×˜0.004±0.001 in.) while still making electrical continuity, therebyeliminating the need for an underlying conductive elastomer on the PCboard. Elastomers do not perform well at high temperatures (above 85 C),thus, in an exemplary embodiment, the PC board substrate is fabricatedfrom a flexible dielectric material such as Kapton® polyimide filmmanufactured by DuPont. The present interposers may be fabricated fromcertain polyimides and other materials that can provide satisfactoryhigh-temperature performance at up to 150 C with peak operation at 200C.

FIG. 1A is an exemplary diagram showing a top view of part of a printedcircuit board 110 including a plurality of interposers 100 located onthe PC board substrate 101. The configuration of interposers 100 shownin FIG. 1A is amenable for use with a ball grid array (BGA), land gridarray (LGA), QFN (Quad Flat package No leads), and any other leadlesschip packages for testing. Most of the latest generation CPUs fromIntel/AMD employ LGA packages. In one embodiment, when, for example, aplurality of interposers 100 are employed with a BGA, the distance ‘d’between adjacent interposer pads is approximately 0.032 inches, or 0.8mm.

FIG. 1B is an exemplary diagram showing a top view of one of theinterposers of FIG. 1A. In an exemplary embodiment, a 3-D process isused to manufacture a printed circuit board 110 which includes aplurality of interposers 100 located on a substrate 101 comprising aKapton® film approximately 0.002″ to 0.003″ thick. This substrate may bemade from other flexible dielectric materials, such as other types ofpolyimide film, or FR-4 (IPC 4101/21), which can be manufacturedsufficiently thin to accommodate a desired amount of flexure and whichcan allow for adherence of a conductive copper trace.

FIG. 2 is a flowchart showing an exemplary process for manufacturing aprinted circuit board in accordance with the present method. FIG. 3A isan exemplary diagram showing a side view of two printed circuit boards,each having a potential cantilever or lever arm in a non-compressedcondition, and FIG. 3B is an exemplary diagram showing the circuitboards of FIG. 1A with the cantilevers in a compressed condition. Anunderstanding of the present process and operation of the interposersmanufactured thereby is facilitated by reference to these drawings (andalso to FIG. 1B) in conjunction with one another.

In accordance with the present method, as shown in FIG. 2, copperconductive traces 104 are initially etched on the upper surface of acopper-clad printed circuit board substrate 101, at step 201. Typically,a substrate 101 has a standard thickness (approximately 0.00075 inch to0.0015 inch) of copper plated to the outside surfaces of the boardsubstrate 101.

At step 205, copper pads 103 are etched on the lower surface of thesubstrate 101. In steps 201 and 205, the copper on each PC board 110 isselectively removed by etching those areas where the copper is notrequired. In the present case, lever arm trace areas 104 on one side ofsubstrate 101 and pad bases 103P on the other side of the substrate arethe only regions where the copper is not etched off of the substrate.Lever arm trace areas 104 and the underlying substrate form cantileverswhen a suitable force is applied, as described below.

For small trace widths, the traces 104 may be deposited directly on thefilm, thus omitting the etching step. The traces (and correspondinglever arms) may be varied in both width and depth to adjust a desiredflex vs. downward compression ratio. In addition, the use of thinnersubstrates (approximately 0.001 inch in thickness and below) allows verylow compression interposers to be fabricated. Depending on overallplanarity of the mating part (the connecting entity) and the PC board(or board-to-board planarity), extremely low compressive forcerequirements and reduction of undesirable modulation of the substratemay be achieved by the use of relatively thinner substrates.

At step 207, vias 107 connecting the traces 104 and corresponding pads103P on the opposite side of substrate 101 may be placed through thesubstrate.

Next, at step 210, an approximately 0.004 inch thick (plus or minusapproximately 0.001″) copper pad plate-up (i.e., trace build-up) 105 isapplied over pad bases 103P and trace areas 103D. For clarity, plate-ups105 on the top and bottom of substrate 101 are respectively designatedas plate-ups 105T and 105B. This selective plate-up forms a lever arm oflength L1 or L2 (shown in FIG. 1B), depending on the length of U-shapedslot 102/102A (described in detail below), which provides the potentialfor compressive displacement and vertical lever arm travel orthogonal tothe plane of substrate 101. Pad plate-up areas 105T/105B provideadditional pad height to allow continued contact with a connectingentity (e.g., a ball grid array) when the entity has been connected to apad and part of the pad is depressed below the upper surface ofsubstrate 101.

In an exemplary embodiment, at step 215, all external surfaces of theplated-up pads 105T/105B are then coated with conductive (e.g., diamond)material for electrical continuity under compression. A conductiveplating process, such as that described in U.S. Pat. No. 6,630,203, isused to coat the pads. This process includes an electrolessco-deposition of metal and hard particles on an electrical contactsurface which enhances the thermal and electrical conductivity betweenthe contact surfaces and their corresponding substrate areas.

At step 220, a ‘U’-shaped slot 102 (hereinafter referred to as a ‘U-cut’} of approximately 0.002 inch, plus or minus approximately 0.001 inch(as indicated by ‘Gw’ in FIG. 1B), is then cut through the Kapton (orother material) substrate, preferably using a laser.

As can be seen from FIG. 1B, in an exemplary embodiment, U-cut 102 hasthe approximate shape of a semicircle with a pair of parallel linesextending tangentially, in the same direction, from opposite ends of thesemicircle. In one embodiment, U-cut 102 is made in an areasemi-circumscribing proximal pad 103P, with extended ends providing alength which is at least greater than the diameter Pd of pad 103P. U-cut102 may thus have an effective length of approximately between 1.5 Pd-2Pd, and a width slightly greater than pad diameter Pd. In an exemplaryembodiment, the distance between the inner edge of U-cut 102 and theouter edge of pad 103P is approximately 0.001 inches, although thisdistance may be smaller or larger, as a function of substrate thickness,rigidity, and other factors.

The length of U-cut 102 is not critical as long as the cut provides alever arm of sufficient length to accommodate a desired amount ofsubstrate/trace flexure. The cut should preferably extend past the pad,approximately half the distance of the lever arm. The ends of U-cut 102may be parallel, as indicated in FIG. 1B, or, for minimum pitcharrangements with a relatively large density of interposers on a givensubstrate, the ends of U-cut 102 may taper inward toward trace 104 in a‘horseshoe’ pattern. As can be seen from FIG. 3B, U-cut 102 allowsopposing pads 103 to flex, by providing compliance between adjacentpads. This laser cutting of the substrate 101 provides for betterflexibility and compliance than that attainable by conventionalcantilevered interposers.

As shown in FIG. 1B, the U-cuts 102 may continue past the closestassociated pad 103P toward (and past) the distal associated pad 103D, asindicated by cut continuation 102A, which provides a lever arm of lengthL2. The length of the U-cut 102 is essentially limited only by thespacing of interposers 100 on substrate 101, i.e., limited by thelocation of an interposer's next nearest pad/cut neighbor.

As shown in FIG. 3B, when a mating force is applied to plate-ups 105T onrespective substrates 101(A) and 101(B) via connecting entities 205(A)and 205(B), as shown by the corresponding arrows, lever arm trace areas104 on respective underlying substrates 101(A) and 101(B) are displacedtoward each other in a cantilevered fashion. The combination of thiscantilevered structure together with pad plate-up area 105 allows forvertical lever arm travel sufficient to allow interposers 100 to provideelectrical continuity between two connecting entities whileaccommodating variations in pad and board height.

FIG. 4 is a diagram showing current flow through an exemplary interposer100. As shown in FIG. 4, when interposers on substrates 101(1) and101(2) are in a compressed condition, the electrical current pathbetween a first connecting entity (e.g., a ball grid array) and a secondconnecting entity (e.g., a circuit under test) is indicated by thedashed line 400A/400B between the top-most plate-up 105T(1) and thebottom-most plate-up 105T(2). Electrical continuity through path400A/400B is established through plate-up 105T(1), via 107(1), trace104(1), plate-up 105B(1), plate-up 105B(2), trace 104(2), via 107(2),and plate-up 105T(2).

FIG. 5 is an exemplary perspective view of the interposer of FIGS. 1 and3A/3B, showing U-cut 102 around plate-up pad 105, and FIG. 6 is anexemplary cutaway view of the interposer of FIG. 5. As shown in FIGS. 5and 6, U-cut 102 has a ‘horseshoe’ shape, which is employed to provideincreased flexibility of lever arm trace areas 104 when the length ofslot 102 must be made relatively short to provide sufficientslot-to-slot spacing between multiple interposers 100. Ahorseshoe-shaped slot 102 thus allows closer interposer spacing whenthere is a high density of interposers 100 on a single substrate 101.

For large-pitched (low board density) designs, a further embodiment ofthe present interposer includes U-shaped cuts made through a substrate101 on both sides of an associated pair of pads 103P/103D. FIG. 7 is anexemplary diagram showing a configuration in which an interposer 700employs two U-shaped slots 102 and 702. As shown in FIG. 7, eachresultant lever arm, indicated by lever trace arm portions P1 and P2,flexes downward (through substrate 101) about line F, on the remainingKapton area bridging the two sides of the interposer 700.

Interposer technology similar to that described above may also beemployed with a single circuit board by installing a single or dualinterposer set 100/700 on the circuit board substrate 101, whileapplying compressive force to electrically mate the interposer with theconnecting entity.

Certain changes may be made in the above methods and systems withoutdeparting from the scope of that which is described herein. It is to benoted that all matter contained in the above description or shown in theaccompanying drawings is to be interpreted as illustrative and not in alimiting sense. For example, the interposers shown in the drawings mayinclude different components than those shown therein. The elements andsteps shown in the present drawings may be modified in accordance withthe methods described herein, and the steps shown therein may besequenced in other configurations without departing from the spirit ofthe system thus described. The following claims are intended to coverall generic and specific features described herein, as well as allstatements of the scope of the present method, system and structure,which, as a matter of language, might be said to fall there between.

What is claimed is:
 1. An electronic module-to-module interconnectingstructure comprising: a first module and a second module, each having adielectric substrate having an first surface and a second surface; acopper trace deposited on the second surface of the substrate, thecopper trace having proximal and distal ends; a copper pad, disposed onthe first surface of the substrate directly upon the proximal end of thetrace; a first copper plate-up area deposited on the pad; a secondcopper plate-up area deposited on the distal end of the trace; a slotpassing through the substrate, wherein the middle of the slot isdisposed in closely-spaced proximity to the distal end of the trace toform a lever arm, and wherein the slot semi-circumscribes the pad andextends on both sides of the trace toward the distal end of the trace;and means for placing the copper pad in electric communication with thecopper trace; the first module and the second module being placed in anorientation such that opposing forces placed upon each lever arm at thedistal end results in opposing deflections of the respective lever armstowards one another in cantilever fashion to drive the first copperplate-up area on the proximate end of the lever arm of the of the firstmodule into electrical contact with the first copper plate up area onthe proximate end of the second module.
 2. The interconnecting structureof claim 1, wherein the slot is of a relatively uniform width betweenapproximately 0.001 inches and 0.003 inches.
 3. The interconnectingstructure of claim 1, wherein the substrate is made from the group ofmaterials consisting of polyamide film and FR-4 material.
 4. Theinterconnecting structure of claim 1, wherein each of the copperplate-up areas has a thickness of between approximately 0.003 inches and0.005 inches.
 5. The interconnecting structure of claim 1, wherein thedistance between the inner edge of the slot and the outer edge of thepad is at least approximately 0.001 inch.
 6. The interconnectingstructure of claim 1, wherein the slot extends a distance of at leasthalf the length of the trace.
 7. The interconnecting structure of claim1, wherein: the proximal end of the trace is displaced in a cantileveredmanner below the lower side of the substrate when a force is appliedproximate to and orthogonal to the pad, the first plate-up area contactsa first connecting entity, and the second plate-up area contacts asecond connecting entity to establish electrical continuitytherebetween, via the first plate-up area, the trace, and the secondplate-up area.
 8. The interconnecting structure of claim 7, wherein thesecond connecting entity is a second interconnecting structuresubstantially identical to the interconnecting structure of claim 1,co-disposed in a mirror-image spatial relationship.
 9. A plurality ofthe interconnecting structures of claim 1, disposed on a single saidsubstrate.